fix: deduplicate glob results in snap_diff:clean rake task (#178)

Overlapping globs (*.diff.png matches *.base.diff.png and *.heatmap.diff.png)
produced duplicate paths, causing the "Removed N" count to over-report.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pftg

Rakefile

@@ -40,7 +40,7 @@ task "snap_diff:clean" do
   patterns = ["**/*.diff.png", "**/*.base.diff.png", "**/*.heatmap.diff.png",
     "**/*.diff.webp", "**/*.base.diff.webp", "**/*.heatmap.diff.webp",
     "**/snap_diff_report.html"]
-  removed = patterns.flat_map { |p| Dir.glob("tmp/#{p}") + Dir.glob("doc/screenshots/#{p}") }
+  removed = patterns.flat_map { |p| Dir.glob("tmp/#{p}") + Dir.glob("doc/screenshots/#{p}") }.uniq
   removed.each { |f| FileUtils.rm_f(f) }
   puts "Removed #{removed.size} diff artifacts"
 end